I can only speculate, but the decision may have been made by the choice of text: Christus factus est pro nobis obediens usque ad mortem, mortem autem crucis. In the Ordinary Form this Gradual is sung on Palm Sunday and Good Friday immediately before the proclamation of the Passion, which it anticipates, functioning much as the Verse or Acclamation before the Gospel does generally in the novus ordo. On Palm Sunday it also continues the theme of the 2nd Reading from Philippians. Perhaps since this chant, taken from the Mass of Holy Thursday, was seen as more fitting to the celebration of the Lord's Passion, it was moved to Palm Sunday and Good Friday, where it replaced the tract Eripe me, Domine from Psalm 139(140). I think textual considerations were paramount in the choice, and not considerations of form or genre.